The proposed research will utilize a pair of identified serotonergic neurons in the terrestrial mollusc Limax maximus to investigate the function of serotonergic synapses at both the behavioral and neurophysiological levels. The neurotransmitter serotonin is used by vertebrates and invertebrates to control an array of post-synaptic structures using a small, discrete population of neurons. The molluscan system available in Limax allows a direct test of the modulatory function of serotonin, i.e., serotonin's ability to control the efficacy of other parallel synpapses and thereby set the level of central excitability for a type of behavior. Physiological experiments will be conducted on a preparation of lips and CNS so that feeding motor output can be triggered by lip chemostimulation. During feeding motor output simultaneous intracellular recordings are made from the identified serotonergic neurons and extracellular recordings from the nerve roots to feeding muscles. Serotonergic neural outflow can be interacted with other neural inputs to the feeding network in order to study the modulatory role of serotonin at the cellular and network levels. Behavioral experiments on intact animals will further characterize the parameters determining bite frequency and bite force.